1. Field of the Invention
The present invention relates to an image recording apparatus, an image processing apparatus, an image processing method and a computer-readable medium, and more particularly, to an image recording apparatus, an image processing apparatus, an image processing method and a computer-readable medium which correct density non-uniformities caused by variations in the characteristics with respect to each of recording elements when an image is recorded on a recording medium (recording paper) using a recording head in which a plurality of recording elements are provided.
2. Description of the Related Art
Japanese Patent Application Publication No. 2003-136764 discloses an inkjet recording apparatus which records an image by ejecting ink onto a recording medium using a recording head in which a plurality of nozzles for ejecting ink are arranged, wherein a pattern for measuring the recording characteristics of the recording head is output, ejection failure nozzles (non-ejection nozzles) which are in a state of ejection failure (non-ejection), of the plurality of nozzles, are judged on the basis of the results of measuring the density of the pattern, the density distribution corresponding to the respective nozzles is determined, the density distribution is subjected to convolution integration using VTF (Visual Transfer Function) or PSF (Point Spread Function), whereupon the result of the portion of the density distribution corresponding to an ejection failure nozzle is compared with a predetermined reference set value, a compensation table for performing compensation using a different color to that of the ejection failure nozzle is determined for each nozzle, and the image data corresponding to an ejection failure nozzle is converted to data of a different color which is to be ejected from another head, using the compensation table.
In the technology described in Japanese Patent Application Publication No. 2003-136764, if an inexpensive scanner having low resolution is used, then the density of the portion of the ejection failure nozzle affects the measured density of the neighboring nozzles, and ejection failure correction and shading correction (density correction) are not carried out correctly. Furthermore, since ejection failures result in a readily visible image defect, then in the case of multiple-sheet printing, and the like, in particular, image processing for making ejection failures less readily visible is carried out rapidly and the output of prints containing defects ought to be reduced. However, with the technology described in Japanese Patent Application Publication No. 2003-136764, it takes time to calculate the correction values. Moreover, in a case where ejection failure correction is carried out by using ink of a different color, there are possibilities that the types of inks of different colors are restricted, for instance, only C, M and Y inks in the case of K ink, and the color tone is changed by the correction process.
One method for rapidly correcting image defects caused by the occurrence of ejection failure is a method in which an independent ejection failure correction device is provided, ejection failures are inspected for each output media, and the results of this inspection are reflected in the subsequent outputs. In other words, it is possible to correct ejection failures immediately, by additionally outputting an ejection failure determination pattern at a specific position of the output image, measuring the ejection failure determination pattern thus output, and inputting this measured ejection failure information to the ejection failure correction device.
Furthermore, a method is also known according to which a density measurement test chart is output, the output chart is measured to acquire density measurement values, density correction values are calculated on the basis of the acquired density measurement values, and the density of image data is corrected by using the calculated density correction values.
However, if this density correction is carried out in conjunction with the ejection failure correction described above, then the ejection failure correction and the density correction are duplicated, and over-correction may occur.